Add doc for enbsubst.py

tests/test-infrastructure/scripts/envsubst.py

@@ -1,8 +1,21 @@
 #!/usr/bin/python3
+"""
+NAME
+       envsubst.py - substitutes environment variables in bash format strings
+
+DESCRIPTION
+    envsubst.py is upgrade of POSIX command `envsubst`
+
+    supported syntax:
+      normal       - ${VARIABLE1}
+      with default - ${VARIABLE1:-somevalue}
+"""
+
 import os
 import re
 import sys
 
+
 def envsubst(template_str, env=os.environ):
     """Substitute environment variables in the template string, supporting default values."""
     pattern = re.compile(r'\$\{([^}:\s]+)(?::-(.*?))?\}')
@@ -14,6 +27,7 @@ def replace(match):
 
     return pattern.sub(replace, template_str)
 
+
 def main():
     if len(sys.argv) > 2:
         print("Usage: python envsubst.py [template_file]")
@@ -30,5 +44,6 @@ def main():
 
     print(result)
 
+
 if __name__ == "__main__":
     main()